This invention relates generally to an electron source and more particularly to a cold electron source for supplying electrons which can collide with gas molecules to form ions or ion species in vacuum gauges and analytical instruments.
Electron emitters used in electron sources are primarily hot filaments which must be operated at temperatures ranging from 1000 to 2200 degrees Celsius. The heating of the cathode requires time and causes gasses to be desorbed from the cathode surface. These gases degrade the measurement environment, particularly in vacuum gauges.
Electron sources using cold emission emitters are known. The electron emission mechanism is electric field assisted tunneling at the surface (field emission) which has the characteristic that the density of the emitted current is determined solely by the electric field at the emitter surface. Since the emitted current is independent of temperature, the full current capacity of the emitter is available at or below room temperature, hence, it is termed a cold cathode or emitter. Current is emitted from the surface when electric field at the surface exceeds a certain value, termed the threshold field. The emitted current increases as the electric field is increased above the threshold field. U.S. Pat. No. 5,278,518 describes an ionization gauge using a cold micropoint cathode. However, such emitters are costly due to the multiple lithographic processes required to produce them. In addition, they have limited operational life when operated at pressures higher than 10xe2x88x926 Torr.
U.S. Pat. Nos. 5,869,922 and 6,042,900 describe emitter surfaces comprising nano-diamond and nano-carbon structures and their formation. J. L. Kwo et. al., J. Vac. Sci. Technol. B 19(1), Jan/Feb 2001, 23-25, and Young Joon Yoon et. al., J. Vac. Sci. Technol. B 19 (1), Jan/Feb 2001, 26-31 describe carbon nanotubes and their formation suitable for use as cold electron emitters. These emitters are highly efficient, easy to manufacture and operate at higher pressures for considerable periods.
It is an object of the present invention to provide an electron source employing emitters having emitting films of nanocrystalline graphite and diamond, carbon nanotubes and combinations thereof, herein nanostructured carbon films.
It is another object of the present invention to provide an inexpensive, simple and efficient electron source.
The foregoing and other objects of the invention are achieved with an electron source which includes an emitter having a nanostructured carbon emitting film and a closely spaced grid for applying electric fields to the film to cause electron emission and acceleration of the emitted electrons through the grid.